Cylinder liners for use in internal combustion engines are static components composing the structure of the engine block, providing to the assembly a closed system for the gases in expansion and promoting the thermal exchange of the heat generated in the combustion with water (wet cylinder liner) or air (dry cylinder liner) circulating around the same.
Among the objectives of the different types of liners are emphasised the maintenance of the sealing of the combustion chamber, the realisation of the thermal exchange of the heat generated within the combustion chamber with the cooling medium (water or air) and the possibility of the reutilisation of the engine block.
In the functioning thereof, an internal combustion engine admits a mixture of air/fuel into the cylinder, entering into spontaneous combustion following being compressed (Diesel engines) or by means of an ignition spark created within the combustion chamber during the compression of the mixture (alcohol and/or petrol engines).
The combustion of the gases in expansion will occur within a closed system such that part of the energy generated impels a piston of the engine downwards and, in this manner, successively, moves the crankshaft, consequently transforming the energy into movement. Thus the cylinder liners act in the functioning of the engine providing the system with the closed condition necessary to the process of energy conversion.
The growing demands in relation to internal combustion engines call for continuous enhancements in relation to the diverse components thereof and the working surfaces thereof. A precise relationship between the cylinder liners, the pistons and the piston rings leads to the improvement of the performance of the engine.
Usually, the cylinder liners for use in internal combustion engines are produced in cast iron with the addition of alloying elements to improve the mechanical and thermal properties thereof. As well as the addition of alloying elements, the liners of cast iron also require optimised working surfaces, contributing to the reduction in the consumption of oil and the recirculation of the gases, producing less particles due to the wear and permitting shorter running in times and, thus, a longer working life.
Consequently, to achieve the optimisation of the working surface of the cylinder liners honing processes are usually carried out, a machining method wherein the tool realises reciprocating and rotating movements, ensuring the cylindricity of the liner and the uniformity of the surface thereof. A honing process properly realised ensures positive effects on the wear of the piston ring, on the emission of particles, on the consumption of oil and on the friction.
Diverse techniques have been developed having the objective of obtaining better conditions of operation of the cylinder liners through varied honing processes.
A first development is revealed in the German document DE102006057111, of the present applicant, relating to a cylinder liner wherein the roughness varies along the direction of displacement of the piston in the interior thereof. More specifically, the portion near the end of the maximum travel of the piston in the direction of the head possesses a region having a first roughness, and the central region of the liner, in terms of the travel of the piston, presents a second roughness, wherein in the intermediate portion of the working surface of the liner the value of roughness is greater than that of the extremities. The focus of the said document resides solely in the specifications of values of roughness of the liner and the distribution of said roughness along the length within the interior of the liner is not specified.
The patent document DE102009010791 reveals a cylinder liner provided with a higher value of roughness at the extremities in relation to the value of roughness in the central region. However, the process utilised for the obtainment of such structures leads to the emergence of recesses having greater and lesser depth (the depth varies considerably therebetween), diminishing the potential efficacy of this solution as pockets of accumulation of lubricating oil. Again, the focus of this document resides solely in the specifications of values of roughness of the liner.
The patent document DE19605588 relates to a cylinder liner the internal surface whereof presents greater values of roughness at the extremities in relation to the second portion. But in order for this result to be achieved the honing process was utilised with certain technical parameters to treat the surface of the extremity regions, utilising different parameters in the central region. In the same manner, the focus of this document resides solely in the specifications of values of roughness of the liner.
Finally, the patent document FR2884889 relates to a cylinder liner the sliding surface whereof presents three values of roughness, being (i) a first value S1, lowest (least rough) in the portion of the extremity facing the head, in the second portion and in the portion of the opposite extremity, (ii) a second value of roughness S2 of medium roughness applied between the extremity of the portion facing the head and the second portion and, finally, (iii) a third greater value of roughness S3 applied between the second portion and the portion of the extremity opposed to that facing the head. In this case also the focus resides solely in the specifications of values of roughness of the liner.
In this manner it may be observed that in the state of the art there exist diverse technologies applied to the processes of honing for cylinder liners, having the particular objective of specifications of values of roughness along the length of displacement of the piston within the interior thereof.
However, documents are not found demonstrating, in addition to the magnitude of the values of roughness, the ratio of length in terms of variation of the roughnesses obtained in different defined portions of the working surface of the liners.
Having a view to reducing wear through friction and scuffing during the operation of the engine, a cylinder liner for an internal combustion engine is presented provided with a hollow cylindrical body comprising an internal working surface provided with a longitudinal/axial length and divided into at least two portions along the longitudinal length thereof, each of said portions comprising a specific value of roughness and a predefined length over an established interval.